Carbon dioxide (CO2) was the big driver that ended the last Ice Age, scientists said on Wednesday in a study that undermines a key argument by global-warming sceptics.

About 10,000-20,000 years ago, Earth started to emerge from a quarter million years of deep freeze as the terrestrial ice sheet rolled back and warmer temperatures helped Man to spread out and conquer the planet.

What caused the end of this age, known as the Pleistocene, has long been debated.

Until now, the main evidence has come from ice cores drilled in Antarctica whose air bubbles are a tiny time capsule of our climate past.

Traces of CO2 -- the principal greenhouse gas that traps solar heat -- show that carbon concentrations in the atmosphere rose after temperatures were on the rise, not before.

The timing has been seized upon by sceptics as proof that man-made carbon gases either do not cause global warming or at least do not make it as bad as mainstream scientists say.

Instead, natural changes in Earth's orbit, bringing the planet closer to the Sun, caused the warming, according to the dissident view.

But the new study says a far wider picture shows orbital change merely started things going.

Real responsibility for warming lay with CO2, it contends.

"Orbital changes are the pacemaker. They're the trigger, but they don't get you too far," said researcher Jeremy Shakun of Harvard University.

"Our study shows that CO2 was a much more important factor and was really driving worldwide warming during the last deglaciation."

Published in the British journal Nature, the investigation looked at 80 ice cores and sedimentary samples taken from Greenland, lake bottoms and sea floors on every continent.

"Putting all of these records together into a reconstruction of global temperatures shows a beautiful correlation with rising CO2 at the end of the Ice Age," said Shakun.

A rise in carbon dioxide "actually precedes global temperature range, which is what you would expect if CO2 is causing the warming."

The scientists theorise that orbital shift boosted sunlight that warmed the northern hemisphere, causing some of its icesheet to melt and spill gigatonnes of chilly freshwater into the North Atlantic.

The big gush had a dampening effect on a well-known "conveyor belt" of current by which warm water travels northwards on the surface of the Atlantic before cooling and returning southwards at depth.

When the current braked, warm water began to build up in the southern Atlantic, where it swiftly started to warm up Antarctica and the Southern Ocean.

Warming the south in turn shifted the wind and melted sea ice, releasing some of the vast amounts of CO2 that had been absorbed by the ocean and stored in its depths, according to their hypothesis.

Today, CO2 -- disgorged by the burning of coal, oil and gas -- is again in the frame.

In London last week, 20 winners of the Blue Planet Prize, one of the world's most prestigious green awards, said current emissions of warming gases were so high there was only a "50-50" chance of limiting the temperature rise to three degrees Celsius (5.4 degrees Fahrenheit).

There were "serious risks" of a 5 C (9.0 F) rise, a temperature last seen on the planet 30 million years ago, they said.

"CO2 was a big part of bringing the world out of the last Ice Age and it took about 10,000 years to do it," said Shakun.

"Now CO2 levels are rising again, but this time an equivalent increase in CO2 has occurred in only about 200 years, and there are clear signs that the planet is already beginning to respond."

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